Prescribed burning beneficial or neutral for native herbaceous vegetation in an invader dominated commercially grazed savanna

Abstract

Prescribed burning is frequently used in savanna vegetation in Australia and worldwide. In north-east Queensland savanna burning has been proposed for the control of woody weeds including Cryptostegia grandiflora (rubbervine), an invasive shrub/vine of riparian savanna. However, burning as a management tool can have non-target impacts on ecosystems, particularly where associated with additional invasive species, such as the exotic stoloniferous grass species Bothriochloa pertusa (Indian couch). For this reason, an experiment was established to test for non-target impacts of prescribed burning on herbaceous vegetation. The experiment consisted of wet and dry season burning, and single and double burning treatments during a 4 year study. Annual herbaceous surveys were conducted, both before treatments applied, and then annually. We addressed alternative hypotheses, that (i) burning would exacerbate grazing impacts resulting in negative outcomes for native herbaceous vegetation, or alternatively (ii) burning would be neutral or beneficial to native herbaceous assemblages due to savanna co-evolution with fire. Burning treatments resulted in subtle but beneficial responses for native herbaceous vegetation. Native legume biomass, percent composition and percent frequency, and forb species richness, increased 4–5 months following wet season burning with above average rainfall. There was a delayed (8–18 months) positive response of burning (both wet and dry season) in percent composition of native grazing sensitive perennial grasses. There was a slight short-term decline in introduced perennial grass percent composition and biomass following treatment. Results were consistent with predominantly positive or neutral responses to prescribed burning. There was no evidence to support the hypothesis that invaded native savanna herbaceous assemblages declined with burning treatment applications. These results are discussed in relation to other savanna studies in northern Australia and to the role of fire in the maintenance of herbaceous vegetation in savanna biomes.